The present invention generally relates to manganese dry cells and more particularly, to acetylene black in a cathode mixture of the manganese dry cells.
In conventional dry cells, acetylene black stipulated in JIS (Japanese Industrial Standards) K-1469 "Acetylene Black" 5.5 as such physical properties as a specific surface area of 40-65 m.sup.2 /g and a quantity of absorbed hydrochloric acid of 14-18 cc. FIG. 1 shows relation between the specific surface area and the quantity of absorbed hydrochloric acid. As shown in FIG. 1, since an interrelation exists between the quantity of absorbed hydrochloric acid and the specific surface area of acetylene black, only the specific surface area of acetylene black is described, hereinbelow.
In a dry cell in which acetylene black acts as conductor and electrolyte consisting mainly of zinc chloride is used, mixing ratio in weight of manganese dioxide (MnO.sub.2) to acetylene black, chereinbelow, referred to as "MmO.sub.2a /AB ratio"), ranges between 4/1 and 7/1 in practical use.
In electric discharge tests of a dry cell employing such known acetylene black, the dry cell connected to a load of 2 .OMEGA. is subjected to electric discharge for 30 min. at an interval of reset time or not less than 8 hours twice a day (hereinbelow, referred to as "2-.OMEGA. load type intermittent electric discharge") as stipulated in JIS. At this time, by setting a final discharge voltage to 0.9 V, FIGS. 2a and 2b show, respectively, total discharge duration and discharge efficiency expressed by ratio of actual discharge capacity to theoretical discharge capacity set under identical conditions. As will be seen from FIG. 2b, experiments on R20 (IEC Standards) type unit manganese dry cells have revealed that discharge performance deteriorates sharply at MnO.sub.2 /AB ratio of 8/1 or more and discharge efficiency also drops below 50% at MnO.sub.2 /AB ratio of 7/1 or less in the dry cells employing conventional acetylene black. Thus, if the known acetylene black is employed, MnO.sub.2 /AB ratio is limited to 7/1 at its maximum. Meanwhile, if acetylene black having a large specific surface area is employed, its discharge efficiency at MnO.sub.2 /AB ratio higher than 7/1 exceeds that obtained at the limit MnO.sub.2 /AB ratio of 7/1 in the known acetylene black. However, in this case, discharge capacity of a dry cell employing this acetylene black during its storage at a high temperature of 45.degree. C. deteriorates excessively.